Method and apparatus to improve frequency stability of an integrated circuit oscillator

ABSTRACT

An apparatus providing a simple low cost integrated circuit oscillator with improved frequency stability over a range of selected frequencies by reducing the impact of process and temperature variations on a base current of bipolar transistor of the integrated circuit oscillator. A circuit includes a capacitor coupled to be alternatingly charged and discharged by first and second current sources. A first voltage follower circuit including a first bipolar transistor having a base is coupled to the capacitor. The first bipolar transistor is biased such that a voltage at an emitter of the first bipolar transistor follows a voltage on the capacitor. A current mirror having first and second current paths is included. The first current path is coupled to the base of the first bipolar transistor. The first current path provides substantially all of a base current received by the base of the first bipolar transistor. A second voltage follower circuit including a second bipolar transistor having a base coupled to the second current path is included. The second current path provides substantially all of a base current received by the base of the second bipolar transistor.

REFERENCE TO PRIOR APPLICATION

This application is a continuation of and claims priority to U.S.application Ser. No. 10/717,228, filed Nov. 19, 2003, now pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to integrated circuits, and morespecifically, the present invention relates to integrated circuits thatare controllers for switching power supplies.

2. Background Information

A large class of switching power supplies operates with a fixedswitching frequency. It is often desirable to know that the switchingfrequency will not deviate by more than a specified amount from anominal value during normal operation of the power supply. Suchknowledge is very useful to designers because it allows them to selectoptimum components for the power supply and for the system that mustoperate with it.

Designers choose a switching frequency that is suitable for theparticular application. The selection of frequency depends typically onthe amount of power to be processed and the topology of the powerconverter. Various other parameters and specifications that areimportant to the use of the power supply also influence the selection ofits switching frequency.

The controllers for modern switching power supplies are typicallyintegrated circuits. Some integrated circuit controllers have only onefixed switching frequency, whereas others offer the designer a choice oftwo or more fixed switching frequencies. The controllers that haveoptions for more than one fixed frequency typically allow the designerto select the desired frequency by way of a particular connection ofterminals on the integrated circuit.

SUMMARY OF THE INVENTION

Disclosed are methods and apparatuses to reduce the difference betweenthe actual frequency and the desired frequency of a simple low costoscillator in an integrated circuit. In one embodiment, the oscillatorgenerates a sawtooth voltage waveform by changing the voltage on acapacitor between two thresholds. The capacitor is part of theintegrated circuit. Current sources add and remove electric charge onthe capacitor to change its voltage between the thresholds. Thefrequency of the oscillator depends on the currents from the currentsources that add and remove the electric charge on the capacitor. Thecurrent sources are designed with ordinary techniques for temperaturecompensation to reduce variations with temperature. The capacitor iscoupled to the base of a first bipolar transistor, the emitter and thecollector of the bipolar transistor coupled to other devices in theintegrated circuit. A second bipolar transistor, substantially the sameas the first bipolar transistor, is coupled to have the same basecurrent as the first bipolar transistor. The base current of the secondbipolar transistor is coupled to a current mirror circuit that addscurrent equivalent to the base current of the second bipolar transistorto the base current of the first bipolar transistor. Thus, the basecurrent required by the first bipolar transistor comes from the currentmirror and not from the capacitor. Hence, variations in the base currentof the first transistor over the range of operating temperature do notsubstantially alter the charge of the capacitor to change the frequencyof the oscillator.

Additional features and benefits of the present invention will becomeapparent from the detailed description, figures and claims set forthbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying Figures.

FIG. 1A is diagram that shows the general functional elements of asimple oscillator that is suitable for the controller of a switchingpower supply.

FIG. 1B is diagram that shows the waveforms associated with the elementsof the simple oscillator illustrated in FIG. 1A.

FIG. 2 is a diagram that shows a section of the oscillator of FIG. 1Ashowing a how a bipolar transistor is used for the voltage followerfunction.

FIG. 3 is a diagram showing one embodiment of a section of an oscillatorin accordance with the teachings of the present invention.

FIG. 4 is a diagram showing one embodiment of how a plurality of currentsources can be switched to select different frequencies and duty ratiosfor an oscillator in accordance with the teachings of the presentinvention.

FIG. 5 is a diagram showing one embodiment of a switching power supplywith an integrated circuit controller including one embodiment of anoscillator in accordance with the teachings of the present invention.

DETAILED DESCRIPTION

An embodiment of a method to improve the stability of the nominalfrequency of an integrated circuit oscillator over a wide range ofnominal frequencies, temperature variations and process variations isdisclosed. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one having ordinary skill inthe art that the specific detail need not be employed to practice thepresent invention. Well-known methods related to the implementation havenot been described in detail in order to avoid obscuring the presentinvention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, the appearances of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout this specification are not necessarily all referring to thesame embodiment. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

An objective in the design of integrated circuit controllers forswitching power supplies is to achieve the desired performance at thelowest cost. Use of simple circuits that require the least possiblesemiconductor material is key to a low cost design. Therefore, it isdesirable to generate all switching frequencies with one simpleoscillator circuit. It is also desirable for the oscillator to minimizethe deviation of each fixed frequency from a specified nominal valueover the range of operating temperature and variations in themanufacturing process.

In general, the oscillator is coupled to other circuits in theintegrated circuit. Special techniques are required to prevent thevariations of parameters of circuits coupled to the oscillator fromaltering the frequency of the oscillator without sacrificing desiredperformance at the lowest possible cost. Variations of parameters canoccur from changes in temperature and from tolerances of themanufacturing process.

Embodiments of the present invention involve methods and apparatuses toreduce the variation in frequency of the oscillator from changes intemperature and process variations over a range of selected nominalfrequencies without substantial increase in complexity or sacrifice toperformance.

FIG. 1A shows a typical arrangement of a simple integrated circuitoscillator that is commonly used in control circuits for switching powersupplies. As illustrated, a single pole single throw switch 103 iscontrolled by a comparator 107. It will be appreciated by one skilled inthe art that switch 103 in FIG. 1A represents the function of anequivalent mechanical switch that is implemented with appropriatesemiconductor devices such as for example a transistor in the integratedcircuit. In one embodiment, comparator 107 is implemented with an inputhaving hysteresis to give an upper threshold voltage and a lowerthreshold voltage. The design and operation of a comparator withhysteresis will be familiar to one skilled in the art.

As shown in the illustrated embodiment, comparator 107 has an inputvoltage V_(F) on line 106 and an output voltage V_(O) on line 108. Allvoltages are with respect to the ground reference 111. The comparator107 changes the state of its output voltage V_(O) from a high state to alow state when the voltage at its input 106 rises above an upperthreshold V_(UTH). The comparator changes the state of its outputvoltage V_(O) from a low state to a high state when the voltage at itsinput 106 falls below a lower threshold V_(LTH). To illustrate, oneembodiment of the output voltage V_(O) and the voltage V_(F) isillustrated in FIG. 1B oscillating between V_(UTH) and V_(LTH). Asillustrated in FIG. 1B, the voltage V_(F) waveform is a sawtoothwaveform oscillating between V_(LTH) and V_(URH) and V_(O) is a waveformoscillating between LOW and HIGH in one embodiment. The output V_(O) ofthe comparator 107 is coupled to the single pole single throw switch 103by line 108. The switch 103 is in its open state when the voltage online 108 is at its high state. The switch 103 is in its closed statewhen the voltage on line 108 is at its low state. The input to thecomparator 107 on the line 106 is the voltage V_(F) that is also theoutput of voltage follower 105.

In the embodiment illustrated in FIG. 1, the purpose of the voltagefollower 105 is to keep the voltage V_(F) at its output on line 106substantially equal to the voltage V_(C) at its input on line 102, whileconducting negligible current 109 from the capacitor 101. The state ofthe switch 103 causes the voltage on capacitor 101 to change in one oftwo ways. In one embodiment, the current I₀ from current source 100 isconstant. It will be apparent to one skilled in the art that the currentfrom current source 100 can be variable to change the characteristics ofthe oscillator for particular applications. In one embodiment, a cycleof the oscillator starts when switch 103 opens. When switch 103 is open,the current I_(C) 110 into the capacitor is the difference between theconstant current from current source 100 and the current I_(B) 109.Since the current I_(B) 109 is by design nearly constant andsubstantially less than the current I_(O) from the current source 100,the voltages V_(C) at line 102 and V_(F) at line 106 will increase at alinear rate.

When the voltage V_(F) at line 106 reaches the upper threshold V_(UTH)of the comparator, the switch 103 will close. When switch 103 is closed,current I_(C) 110 into the capacitor 101 will become negative becausethe current KI_(O) from current source 104 is greater than the currentI_(O) from current source 100. The current KI_(O) from current source104 is greater than the current from current source 100 by the ratio K.In one embodiment, the ratio K is constant. It will be apparent to oneskilled in the art that in other embodiments the ratio K may be variableto change the characteristics of the oscillator to suit particularapplications.

Voltages V_(C) at line 102 and V_(F) at line 106 will decrease at alinear rate until the voltage V_(F) at line 106 reaches the lowerthreshold of comparator 107, causing switch 103 to open. The cycle thenrepeats when switch 103 opens. The frequency of the oscillator is therate at which the cycle repeats. Larger values of the current I_(O) willproduce higher frequencies. The duty ratio of the oscillator is thefraction of one cycle that corresponds to the time switch 103 is open.For a given value of K, the oscillator will have the same duty ratio forall values of the current I_(O).

In one embodiment, the voltage follower 105 includes a single NPNbipolar transistor with a current source in the emitter. FIG. 2illustrates one embodiment of a typical single transistor implementationof voltage follower 105. As shown in FIG. 2, the voltage follower 200 inFIG. 2 includes bipolar NPN transistor 201 and emitter bias currentsource 202. A single transistor implementation of the voltage followermay use either a bipolar transistor or a field effect transistor.

An advantage of using a bipolar transistor in this embodiment instead ofusing a field effect transistor is that a field effect transistor isgenerally too slow to give the desired performance. A disadvantage ofusing the bipolar transistor for the voltage follower is that thecurrent I_(B) 109 into the base of transistor 209 usually cannot be madesmall enough to be negligible. The undesirable effect of current I_(B)109 is that it changes the frequency of the oscillator from the desirednominal value. The current I_(B) 109 also changes its valuesignificantly with temperature because it is the base current of abipolar transistor, as will be familiar to one skilled in the art. Theimpacts of these undesirable effects are greater for lower frequenciesof the oscillator because the current I_(B) 109 becomes a largerfraction of the capacitor current I_(C) 110 when the currents I_(O) andKI_(O) of current sources 100 and 103, respectively, are reduced tolower the frequency of the oscillator. It can be shown that thecontribution of the current I_(B) 109 to the fractional change infrequency with respect to the desired nominal value in the embodiment ofFIG. 2 is $\begin{matrix}{\frac{\Delta\quad f}{f_{0}} = {- \left\lbrack {{\left( \frac{I_{B}}{I_{0}} \right)\frac{K - 2}{K - 1}} + {\left( \frac{I_{B}}{I_{0}} \right)^{2}\frac{1}{K - 1}}} \right\rbrack}} & \left( {{Equation}\quad 1} \right)\end{matrix}$where f₀ is the desired nominal frequency, I_(B) is the current 109, I₀is the value of the current source 100, and K is the ratio of currentsource 104 with respect to current source 100, K>1.

In one embodiment, I_(B) is about 0.2 microamperes at room temperature,I₀ is 2.4 microamperes and K is 5. For these values, the presence ofcurrent I_(B) 109 will reduce the actual frequency from the desirednominal frequency by about 6.4% at room temperature. For larger valuesof K with the same I_(B) and I₀, the reduction in frequency approaches8.3%. The impact of this effect will be greater at lower frequenciesthat use lower values of I₀. This change will be in addition to thechange caused by variations in other parameters due to changes intemperature and variations in the manufacturing process. The variationin base current of a bipolar transistor can be large over the range ofoperating temperature, even if the emitter current is constant. It isdesirable, therefore, to reduce or eliminate the influence of currentI_(B) 109 on the frequency of the oscillator. This is accomplished byone embodiment of the present invention, as illustrated for example inFIG. 3.

In one embodiment of a section of an oscillator circuit that isillustrated in FIG. 3, a first voltage follower circuit 200 includes afirst NPN bipolar transistor 201 and a first emitter bias current source202. A second voltage follower circuit 300 includes a second NPN bipolartransistor 301 and a second emitter bias current source 302. In oneembodiment, the second voltage follower circuit 300 is substantially thesame as the first voltage follower circuit 200. Therefore, the basecurrent I_(BF) 304 of the second voltage follower 300 in one embodimentis substantially the same as the base current I_(BF) 306 of the firstvoltage follower 200. The base current I_(BF) 304 is coupled to acurrent mirror 303. One skilled in the art will be familiar with variousimplementations of a current mirror, which is fundamental to the designof integrated circuits. It will also be appreciated by one skilled inthe art that the second bipolar transistor 301 needs to match the firstbipolar transistor 201 only in current density, and not in absolutecurrent magnitude. The current mirror 303 and the second bipolartransistor 301 with its emitter bias current source 302 are designedsuch that the output current I_(BF) 305 of the current mirror 303matches the base current I_(BF) 306 of the first bipolar transistor 201.

The output of the current mirror 303 injects into line 102 an outputcurrent I_(BF) 305, which is substantially the same as current I_(BF)304, which is also substantially the same as the base current I_(BF) 306of the first voltage follower 200. Since the net input current I_(B) 109is the difference between the substantially equal currents 305 and 306,the current I_(B) 109 is substantially zero. The reduction of currentI_(B) 109 to zero effectively eliminates its undesirable influence onthe performance of the oscillator, and permits the use of the low costbipolar transistor solution for the voltage follower function inaccordance with the teachings of the present invention.

One skilled in the art will recognize that the magnitude of the currentof current source 100 and current of current source 104 can be adjustedindependently to change the frequency of the oscillator. One skilled inthe art will also recognize that the ratio of the current of currentsource 100 to the current of current source 104 can be adjusted tochange the frequency and the duty ratio of the oscillator. In oneembodiment, different frequencies and duty ratios are selected by theaddition and removal of current sources as shown by example in FIG. 4.

To illustrate, FIG. 4 is a diagram showing one embodiment of how aplurality of current sources can be switched to select differentfrequencies and duty ratios for an oscillator in accordance with theteachings of the present invention. As shown in the depicted embodiment,closure of double pole single throw switches 402 and 403 in FIG. 4augments the current from current sources 100 and 104, which will varythat rate at which capacitor 101 is alternatingly charged anddischarged, thereby varying the frequency and/or duty ratio of theoscillator. The oscillator operates at its lowest frequency whenswitches 402 and 403 are both open. Four distinct frequencies and dutyratios of the oscillator are possible with the example shown in FIG. 4.One skilled in the art will appreciate that additional current sourcesand switches may be used to achieve a greater number of frequency andduty ratio options. It will also be apparent to one skilled in the artthat the addition of multiple current sources to a single current sourceis functionally equivalent to a change in the magnitude of a singlecurrent source. Therefore, in one embodiment, current source 410 may beconsidered a single variable current source comprised of current sources100, 400 and 401 and switches S1 and S2 while current source 411 may beconsidered a single variable current source comprised of current sources104, 404 and 405 and switches S1 and S2.

In yet another embodiment, an oscillator may be included in anintegrated circuit that controls a switching power supply in accordancewith the teachings of the present invention. To illustrate, FIG. 5 is adiagram of one embodiment of a switching power supply with an integratedcircuit controller including one embodiment of an oscillator inaccordance with the teachings of the present invention. An unregulateddirect current (DC) input voltage V_(IN) 500 is converted to a regulatedDC output voltage V_(OUT) 502 by a switching converter 501 that iscontrolled by an integrated circuit 517. All voltages are with respectto the ground reference 111. The state of a single pole double throwpower switch S_(P) 503 is controlled by the signal PWM_(OUT) 507 fromthe integrated circuit 517.

In operation, switch S_(P) 503 couples the inductor 504 to the inputvoltage V_(IN) 500 when PWM_(OUT) on line 507 is high. Switch S_(P) 503couples one end of the inductor 504 to the ground reference 111 when thesignal PWM_(OUT) on line 507 is low. A capacitor 505 is coupled toinductor 504 and filters the alternating current (AC) current ininductor 504 to provide a substantially DC voltage to a load 506. In oneembodiment, a sawtooth oscillator 514 included in the integrated circuit517 is designed in accordance with the teachings of the presentinvention. The frequency of the sawtooth oscillator 514 withinintegrated circuit controller 517 determines the rate of switching.

In one embodiment, a plurality of functional terminals 511 on theintegrated circuit 517 can be coupled to operate the various functionsof integrated circuit 517 in a desired manner. In one embodiment,functional terminals 511 can set the frequency of the oscillator 514.Integrated circuit 517 senses the output voltage V_(OUT) 502 of theswitching converter 501 at a terminal V_(SENSE) 509 with respect to aground terminal GND 508. In one embodiment, an error amplifier 510within the integrated circuit 517 amplifies the difference between thevoltage at terminal V_(SENSE) 509 and a reference voltage 516 internalto the integrated circuit 517. A comparator 512 compares the errorvoltage output 513 of error amplifier 510 to the sawtooth voltage V_(F)515 that is an output of the oscillator 514. The output 507 of thecomparator 512 is high when the error voltage 513 is greater thansawtooth voltage 515. The output 507 of comparator 512 is low when theerror voltage 513 is less than the sawtooth voltage 515. Thus, theperiodic switching of power switch S_(P) 503 is modulated by theintegrated circuit 517 in a manner to regulate the output voltageV_(OUT) 502.

It will be apparent to one skilled in the art having the benefit of thisdisclosure that many ways are known to implement the function of theswitch S_(P) 503 with semiconductor devices, such as for example twotransistors, or a transistor and a diode. In addition, one skilled inthe art having the benefit of this disclosure will also appreciate thatswitching converter 501 in FIG. 5 is just one example of many differentcircuits that are commonly used in switching power supplies that canemploy an oscillator in accordance with the teachings of the presentinvention.

In the foregoing detailed description, the method and apparatus of thepresent invention have been described with reference to a specificexemplary embodiment thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1. A circuit, comprising: a capacitor coupled to be alternatinglycharged and discharged by first and second current sources; a firstvoltage follower circuit including a first bipolar transistor having abase coupled to the capacitor, the first bipolar transistor biased suchthat a voltage at an emitter of the first bipolar transistor follows avoltage on the capacitor; a current mirror having first and secondcurrent paths, the first current path coupled to the base of the firstbipolar transistor, the first current path providing substantially allof a base current received by the base of the first bipolar transistor;and a second voltage follower circuit including a second bipolartransistor having a base coupled to the second current path, the secondcurrent path providing substantially all of a base current received bythe base of the second bipolar transistor.
 2. The circuit of claim 1wherein the base current received by the base of the first bipolartransistor includes substantially zero current received from thecapacitor.
 3. The circuit of claim 1 wherein the first and secondbipolar transistors are substantially matched in current density.
 4. Thecircuit of claim 1 further comprising first and second bias currentsources, the first bias current source coupled to the emitter of thefirst bipolar transistor and the second bias current source coupled toan emitter of the second bipolar transistor.
 5. The circuit of claim 1further comprising a switch coupled between the second current sourceand the capacitor, the switched coupled to be alternatingly opened andclosed such that when the switch is opened, the first current source iscoupled to charge the capacitor and when the switch is closed, the firstand second current sources are coupled to discharge the capacitor. 6.The circuit of claim 5 wherein the switch is coupled to be opened untilthe voltage on the capacitor is charged to a first threshold and whereinthe switch is coupled to be closed until the voltage on the capacitor isdischarged to a second threshold.
 7. The circuit of claim 6 furthercomprising a comparator coupled to receive the voltage at the emitter ofthe first bipolar transistor, an output of the comparator coupled tocontrol the switch.
 8. The circuit of claim 1 wherein a ratio ofcurrents provided by the first and second current sources issubstantially fixed to provide a substantially fixed duty ratio of anoscillating voltage provided on the capacitor.
 9. The circuit of claim 1wherein a ratio of currents provided by the first and second currentsources is variable to adjust a duty ratio of an oscillating voltageprovided on the capacitor.
 10. The circuit of claim 1 wherein magnitudesof currents provided by the first and second current sources aresubstantially fixed to provide a substantially fixed frequency of anoscillating voltage provided on the capacitor
 11. The circuit of claim 1wherein magnitudes of currents provided by the first and second currentsources are variable to adjust a frequency of an oscillating voltageprovided on the capacitor.